Using The Source SDK for HL2 mapping
 

Quote - " So, you've finished HalfLife2, downloaded the Source SDK but haven't a clue how to set it up and use it to make maps for Half-Life2?
Look no further... on these pages I'll show you, step by step, how to set up Hammer4 for HL2, how to make a basic one-room "map" and run it in HL2.

To be able to test your map without waiting for HL2 to load when you want to run it you may want to start the game first, go to options/keyboard/advanced and tick the developer mode box and apply... then minimise the game so it's ready to go when you are (Alt+Tab).
This is likely to be dependant on your system specs as a low-end system might choke running HL2 & Hammer4 at the same time... my Athlon64 3500 with 1GB of CorsairXMS handles it fine but you will need to use your discretion when it comes to your own system. "

Link - http://www.raptor-mods.co.uk/index.php?article&id=1290